docs: def ssh_user(self, cluster='main'):
          """Return the ssh user for a cluster or current user if undefined."""
          if not self.config.has_section(cluster):
              raise SystemExit("Cluster '%s' not defined in %s"
                               % (cluster, self.config_file))
          try:
              return self.config.get(cluster, 'ssh_user')
          except NoOptionError:
              return pwd.getpwuid(os.getuid()).pw_name